The present invention relates to a sheet feeding device configured to feed sheets, such as document sheets, one by one to a processing section such as an image forming section, as well as a document reading device configured to read an image carried on a document sheet fed by the sheet feeding device. One known image reading device for reading a document image at an image reading section is provided with a sheet feeding device for feeding document sheets one by one to the image reading section. Such a sheet feeding device includes: a sheet support capable of placing plural document sheets thereon; a pickup roller which can move up and down relative to an upper surface of the sheet support; and a separating roller disposed downstream of the pickup roller in a sheet feed direction, as disclosed in Japanese Patent Laid-Open Publication No. HEI 06-215889.
During standby for sheet feeding, the pickup roller moves up to an upper limit position to allow document sheets to be placed on the sheet support. During sheet feeding, the pickup roller moves down to contact the uppermost surface of the document sheets placed on the sheet support and then rotates. The separating roller separates the uppermost one of document sheets picked up by the pickup roller from the rest in order to feed only the uppermost one of the document sheets toward the image reading section.
In usual cases the sheet support receives document sheets thereon after the sheet feeding device has been powered on. In some cases, however, such document sheets are placed on the sheet support again after a paper jam has been eliminated. For this reason, conventional sheet feeding devices are generally configured to move the pickup roller up to the upper limit position during an initial operation performed just after powering on or just after elimination of the paper jam.
It is possible that document sheets to be fed remain on the sheet support at the time of the occurrence of the paper jam. Therefore, the vertical position of the pickup roller in contact with the upper surface of the uppermost document sheet differs depending upon the number of document sheets placed on the sheet support. Accordingly, the distance to be traveled by the pickup roller up to the uppermost position at the time just after the paper jam elimination varies in accordance with the number of document sheets to be fed that remain on the sheet support. If the distance of travel of the pickup roller in moving up the pickup roller is fixedly set to the distance from the upper surface of the sheet support to the upper limit position, the pickup roller collides with other members including an outer cover during its upward movement just after the paper jam elimination to generate an unusual sound when a large number of document sheets remain on the sheet support. In order to obviate such an inconvenience, the sheet feeding device has to be provided with means for detecting the position of the pickup roller. The provision of such means incurs increases in cost and in device size.
A feature of the present invention is to provide a sheet feeding device which is capable of preventing the pickup roller from colliding with other members without the need to provide the means for detecting the position of the pickup roller, as well as a document reading device provided with such a sheet feeding device.